fix(8.0): metadata cold-read guard — no more silent [] on cold find({where})

A downstream deployment reported find({where}) returning a silent [] on a
freshly-opened brain (a native metadata provider that reports data but hasn't
loaded its field postings), blanking filtered pages after every restart. Brainy
had a cold-read guard for GRAPH reads (verifyGraphAdjacencyLive → self-heal or a
loud GraphIndexNotReadyError) but no equivalent for metadata `where`.

Add verifyMetadataLive() — the field-index counterpart, one-shot per brain on the
first filtered find(): probe a known persisted entity's plain field value; if the
index serves it, live (the only cost on a warm brain — one O(1) probe). If not,
the postings didn't load: rebuild from canonical + re-probe; if still unserved,
throw the new exported MetadataIndexNotReadyError rather than let a silent [] pass.
Inconclusive cases (empty store, no plain field, shared-store foreign entity,
migrating provider) resolve to live — never a false rebuild.

The 8.0 open-core JS index already cold-loads correctly (verified: cold where 3/3,
cold related 2/2) — this guards the NATIVE path, where the durable cold-load cure
is cortex-side (same shape as the graph 2.7.8 cure). 4 unit tests (warm no-rebuild,
self-heal, loud-fail, no-filter-no-probe). Gates: typecheck 0, build 0, test:unit
1757/1757.

src/index.ts

@@ -139,7 +139,7 @@ export { EntityNotFoundError, RelationNotFoundError } from './errors/notFound.js
 
 // Base error + typed migration-lock error — thrown by any data-plane call while a
 // brain runs its one-time 7.x→8.0 upgrade; catch to answer HTTP 503 + Retry-After.
-export { BrainyError, MigrationInProgressError, GraphIndexNotReadyError } from './errors/brainyError.js'
+export { BrainyError, MigrationInProgressError, GraphIndexNotReadyError, MetadataIndexNotReadyError } from './errors/brainyError.js'
 export type { BrainyErrorType } from './errors/brainyError.js'
 
 // ============= 8.0 Db API — generational MVCC =============